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A new generation of
high speed, silicon-based information technology has been brought a step closer
by researchers in the Department of Electronic and Electrical Engineering at
UCL and the London Centre for Nanotechnology.

The team’s research, published in
Nature Photonics journal,
provides the first demonstration of an electrically driven, quantum dot laser
grown directly on a silicon substrate (Si) with a wavelength (1300-nm) suitable
for use in telecommunications.

Silicon is the most
widely used material for the fabrication of active devices in electronics.
However, the nature of its atomic structure makes it extremely hard to realise
an efficient light source in this material.

As the speed and
complexity of silicon electronics increases, it is becoming harder to
interconnect large information processing systems using conventional copper
electrical interconnects. For this reason the field of silicon photonics (the
development of optical interconnects for use with silicon electronics) is
becoming increasingly important.

The ideal light
source for silicon photonics would be a semiconductor laser, for high
efficiency, direct interfacing with silicon drive electronics and high-speed
data modulation capability. To date, the most promising approach to a light
source for silicon photonics has been the use of wafer bonding to join compound
semiconductor laser materials from which lasers can be made to a silicon
substrate.

Direct growth of
compound semiconductor laser material on silicon would be an attractive route
to full integration for silicon photonics. However, the large differences in
crystal lattice constant between silicon and compound semiconductors cause
dislocations in the crystal structure that result in low efficiency and short
operating lifetime for semiconductor lasers.

The UCL group has
overcome these difficulties by developing special layers which prevent these
dislocations from reaching the laser layer together with a quantum dot laser
gain layer. This has enabled them to demonstrate an electrically pumped 1,300
nm wavelength laser by direct epitaxial growth on silicon. In a recent paper in
Optics Express (Vol. 19 Issue
12, pp.11381-11386 (2011)) they report an optical output power of over 15 mW per
facet at room temperature.

In related work the
group, working with device fabrication colleagues at the EPSRC National Centre
for III-V Technologies, have demonstrated the first quantum dot laser on a
germanium (Ge) substrate by direct epitaxial growth. The laser, reported in Nature Photonics , (DOI: 10.1038/NPHOTON.2011.120, 12 June 2011) is
capable of continuous operation at temperatures up to 70 deg. C and has a
continuous output power of over 25 mW per facet.

Leader of the
epitaxy research that enabled the creation of these lasers and Royal Society
University Research Fellow in the UCL Department of Electronic and Electrical
Engineering, Dr Huiyun Liu, said: "The use of the quantum dot gain layer
offers improved tolerance to residual dislocations relative to conventional
quantum well structures. Our work on germanium should also permit practical
lasers to be created on the Si/Ge substrates that are an important part of the
roadmap for future silicon technology."

Head of the
Photonics Group in the UCL Department of Electronic and Electrical Engineering,
Principal Investigator in the London Centre for Nanotechnology and Director of
the EPSRC Centre for Doctoral Training in Photonic Systems Development,
Professor Alwyn Seeds, said: "The techniques that we have developed permit
us to realise the Holy Grail of silicon photonics - an efficient, electrically
pumped, semiconductor laser integrated on a silicon substrate. Our future work
will be aimed at combining these lasers with waveguides and drive electronics
leading to a comprehensive technology for the integration of photonics with
silicon electronics."